1. Field of the Invention
The present invention relates to a single-stage zero-current switching driving circuit, and more particularly to a single-stage zero-current switching driving circuit for ultrasonic motor.
2. Description of the Related Art
FIG. 1 shows a circuit diagram of a conventional buck-boost power factor correction circuit. The conventional buck-boost power factor correction circuit 10 comprises a rectification circuit 11, an active switch 12, an inductor 13, a diode 14 and a capacitor 15. In which, the active switch 12 is a power device MOSFET (Metal-oxide-semiconductor Field-effect Transistor), and a control circuit 17 is used to generate a trigger signal (vgs) to drive the active switch 12 at high frequency. For achieving high power factor, the current of the inductor 13 is designed to operate in a discontinuous-conduction mode (DCM), and the switching frequency and the conduction rate of the active switch 12 are fixed during the whole period of AC power source voltage.
FIG. 2 shows waveforms of an inductor current and the trigger signal. In which, dr and Ts represents the conduction rate and the switching period of the active switch 12 respectively, the operation theory is described below:
The input AC voltage is represented by equation (1) below.vs=Vm sin(2πft)  (1)
In which, f represents the frequency of the AC power source voltage, Vm represents the peak value of the AC voltage. In realistic design, the switching frequency fs of the active switch 12 is far greater than the frequency f of the AC power source. Under this assumption, in each high frequency switching period, the rectified voltage (vrec) can be seen as a fixed value.
When 0≦t≦drTs, the inductor current (ip) increases linearly, the equation is shown below:
                                          i            p                    ⁡                      (            t            )                          =                                                                              V                  rec                                ⁡                                  (                  t                  )                                                            L                p                                      ⁢            t            ⁢                                                  ⁢            0                    ≤          t          ≤                                    d              r                        ⁢                          T              s                                                          (        2        )            
When t=drTs, the inductor current reaches the maximum of each switching period, the peak value of the current of the inductor 13 is represented below:
                                          I                          p              ,              peak                                ⁡                      (            t            )                          =                                                            V                m                            ⁢                                                                sin                  (                                      2                    ⁢                    π                    ⁢                                                                                  ⁢                    ft                                    )                                                                                    L              p                                ⁢                      d            r                    ⁢                      T            s                                              (        3        )            
When drTs≦t<Ts, the active switch 12 is closed, the inductor current flows through the flywheel diode 14 to charge the DC-link capacitor 15, the inductor current starts to decrease linearly, the inductor 13 operates in a discontinuous-conduction mode (DCM), so that the inductor current will decrease to zero before next switching period, for the time being the inductor current is represented below:
                                          i            p                    ⁡                      (            t            )                          =                                                            -                                                      V                    dc                                                        L                    p                                                              ⁢                              (                                  t                  -                                                            d                      r                                        ⁢                                          T                      s                                                                      )                                      +                                          I                                  p                  ,                                      peak                    ⁡                                          (                      t                      )                                                                                  ⁢                                                          ⁢                              d                r                            ⁢                              T                s                                              ≤          t          <                      T            s                                              (        4        )            
In which, Vdc represents a DC-link voltage.
Only during the period when the inductor current increases, the input current flows through the active switch 12, and the switch current (the input current) appears to be a sawtooth wave, so that the average value of the input current of each switching period is represented below:
                                          i                          in              ,              avg                                ⁡                      (            t            )                          =                                                            d                r                            ⁢                                                T                  s                                ·                                                      I                                          p                      ,                      peak                                                        ⁡                                      (                    t                    )                                                                                      2              ⁢                              T                s                                              =                                                                      V                  m                                ⁢                                  d                  r                  2                                ⁢                                  T                  s                                                            2                ⁢                                  L                  p                                                      ⁢                                                        sin                (                                  2                  ⁢                                      π                    ⁢                    ft                                                  )                                                                                      (        5        )            
As shown in equation (5), if the conduction rate dr and the switching period of the active switch 12 are fixed in each input power source period, the average current is only related to the input voltage, so that we only need to put a small filter capacitor at the input terminal to eliminate high frequency composition, and the input current will be a sine wave, the average input current is represented below:
                                          i            in                    ⁡                      (            t            )                          =                                                            V                m                            ⁢                              d                r                2                            ⁢                              T                s                                                    2              ⁢                              L                p                                              ⁢                      sin            (                          2              ⁢                              π                ⁢                ft                                      )                                              (        6        )            
As shown in equation (6), the input current follows and is in phase with the wave of the input voltage, so that not only high power factor is accomplished, but also the total harmonic distortion of the input current is restrained to an extremely small amount. In a period of the frequency of the AC power source voltage, the output power is:
                              P          in                =                                            1                              2                ⁢                π                                      ⁢                                          ∫                0                                  2                  ⁢                  π                                            ⁢                                                V                  m                                ⁢                                                      sin                    (                                          2                      ⁢                                              π                        ⁢                        ft                                                              )                                    ·                                                            i                      in                                        ⁡                                          (                      t                      )                                                                      ⁢                                  ⅆ                                      (                                          2                      ⁢                      π                      ⁢                                                                                          ⁢                      ft                                        )                                                                                =                                                    V                m                2                                            4                ⁢                                  L                  p                                                      ⁢                          d              r              2                        ⁢                          T              s                                                          (        7        )            
When the integration of the voltage of the energy storage inductor to time is smaller than zero, the buck-boost power factor correction circuit operates in a discontinuous-conduction mode (DCM).Vm|sin(2πft)|·drTs+Vdc·(1−dr)Ts≦0  (8)
As shown in equation (8), if the buck-boost power factor correction circuit can operates in a discontinuous-conduction mode (DCM) when inputting the peak value of the voltage, the buck-boost power factor correction circuit can surely operate in a discontinuous-conduction mode (DCM) at any value of the input voltage, and therefore the DC-link voltage (Vdc) must be high enough and satisfies the following equation:Vdc≦Vm·dr/(1−dr)  (9)
FIG. 3 shows a circuit diagram of a conventional driving circuit. The conventional driving circuit 20 comprises a low pass filter 21, a rectifier 22, a buck-boost converter 35 and a class E resonant inverter 36. The buck-boost converter 35 comprises a first diode 23, a second diode 24, a first inductor 25, a first capacitor 26, an active switch 27. The class E resonant inverter 36 comprises the first capacitor 26, the active switch 27, a second capacitor 28, a second inductor 29, a third inductor 31, a third capacitor 32. The buck-boost converter 35 and the class E resonant inverter 36 share the first capacitor 26 and the active switch 27.
The conventional driving circuit 20 integrating the buck-boost converter 35 and the class E resonant inverter 36, which was proposed by Ed Deng and Slobodan Ćuk in 1995, has the advantages of having simple circuit structure and good circuit performance. But because of the circuit structure, there's interaction of energy between the buck-boost converter 35 and the class E resonant inverter 36, the input power is not entirely transmitted to the class E resonant inverter 36 by the buck-boost converter 35, and part of the energy is transmitted from the input terminal to the class E resonant inverter 36, and therefore the buck-boost converter 35 can not accomplish the aim of power factor correction.
Moreover, referring to prior related researches, for example, “Genetic Algorithm Control of the Linear Piezoelectric Ceramic Motor Driving System”, proposed by Department of Electrical Engineering, Yuan Ze University in July 2005, and the thesis obtains the invention patent No. I271024, the circuit structure of the patent is that using LC current source to resonate in parallel and generate LLCC resonance circuit supplied by AC voltage. The LLCC resonance technique is utilized to obtain six stage LC resonant inverter for driving voltage of the motor. The most disadvantage of this method is that the circuit is very complicated, the difficulty of controlling the circuit is relatively increased, and the manufacturing cost is largely increased. Therefore, it lacks the value of being commercialized.
Therefore, it is necessary to provide a single-stage zero-current switching driving circuit for ultrasonic motor to solve the above problems.